Most electronic equipment produced today is comprised of one or more printed circuit boards each populated with different types of electronic components interconnected to each other to form one or more functional circuits. Once populated with components, the printed circuit board is subjected to in-circuit testing. In this way, the operation of the functional circuits contained on the circuit board can be verified. During in-circuit testing of printed circuit boards containing digital components, such as logic gates and memory devices, it is often desirable to isolate and test individual components. However, the component being tested ofter receives input signals from one or more other components. Isolation of the component under test can be accomplished by back driving those devices supplying input signals thereto. Back driving of a device is accomplished by either sourcing current to, or sinking current from, the output thereof so that the signal supplied therefrom to the component under test does not vary. In some instances, current must be alternately sourced to, and sunk from, the device to isolate the component under test therefrom. In practice, commercially available testing machines for achieving in-circuit testing of digital circuits are provided with driver circuits for alternately sourcing and sinking current to permit devices on the circuit board to be back driven.
Presently, many digital components are capable of operating at very high frequencies, often as high as 100 MHz or higher. To reliably test such components at their operating frequency, those devices supplying input signals thereto must be back driven at the same frequency. In practice, the driver circuits within conventional testing machines are only capable of back driving devices at frequencies of no greater than 10 MHz. Thus, many digital components cannot be reliably tested at their operating frequency. Accordingly, there is a need for a method of alternately sourcing current to, and sinking current from, a load at very high frequencies.